This invention relates generally to the field of disc drive data storage devices, and more particularly, but not by way of limitation, to an apparatus and method for detecting servo defects in a disc drive in order to provide improved servo control.
Disc drives typically comprise one or more rotatable discs which are selectively magnetized by a corresponding array of selectively positionable read/write heads in order to store data on the surfaces of the discs during a write operation. During a read operation, the heads sense polarity transitions in the magnetization of the discs in order to reconstruct the data which was previously stored on the discs.
As will be recognized, data is stored on the surfaces of the discs on tracks which concentrically extend about the center of the disc surfaces. A closed loop servo system (also known as a servo loop) is typically provided to sense and control the position of the heads relative to the tracks.
One well known type of servo system is referred to as a dedicated servo, wherein one surface of one of the discs is dedicated to servo information and the remaining surfaces of the discs are used to store data. By using the head associated with the dedicated servo surface to read the servo information, the position of this head can be readily controlled (for reference, this head is commonly referred to as a servo head). Further, because all of the heads are nominally aligned by an actuator assembly, control of the position of the servo head results in corresponding control of the position of the remaining heads, also referred to as data heads. An example of a dedicated servo system is disclosed in U.S. Pat. No. 5,262,907 entitled HARD DISC DRIVE WITH IMPROVED SERVO SYSTEM, issued to Duffy et al., assigned to the assignee of the present invention and incorporated herein by reference.
As track densities in recent generations of disc drives have continued to increase, the trend in the industry is to use a second type of servo system commonly referred to as an embedded servo. As will be recognized, use of an embedded servo results in both servo information and user data being intermittently stored on each track, so that each head operates as both a servo head and a data head. Accordingly, as a selected track passes adjacent a head, the servo information read by the head is provided to the servo loop; moreover, as the head passes over data portions of the selected track, the head operates in conjunction with a disc drive read/write channel to read or write data, respectively, to the data portions of the selected track.
The servo loop typically controls the position of the head through the generation of a position error signal (PES) from the servo information. The PES comprises a series of sample values having a magnitude indicative of the relative position of the head with respect to the track. More particularly, during track following in which the head is caused to follow a selected track, the servo system generates the PES from the received servo information and then uses the PES to generate a correction signal which is provided to a power amplifier to control the amount of current through a coil of the actuator assembly. The coil is immersed in a magnetic field of a voice coil motor, so that the current causes the coil, and hence the head, to move relative to the discs in order to adjust the position of the head.
Typically, the PES is presented as a position dependent signal having a magnitude generally indicative of the relative distance between the head and the center of a track and a polarity indicative of the direction of the head with respect to the track center. Thus, it is common for the PES to have normalized values ranging from, for example -1.0 to +1.0 as the head is swept across the track and to have a value of 0 when the head is positioned over the center of the track. It will be recognized that the PES is generated by the servo loop by comparing the relative signal strengths of burst signals generated from precisely located magnetized fields (servo fields) in the servo information on the disc surface.
As discussed more fully in the previously incorporated Duffy et al. U.S. Pat. No. 5,262,907, the servo fields are generally arranged in an offset checkerboard pattern (also referred to as a quadrature pattern) so that, through manipulation of the magnitudes of the burst signals provided to the servo system as the servo fields are read, the relative position of the head to a particular track center can be determined and controlled. More particularly, samples of digital representations of the analog burst signals are typically provided to a servo loop microprocessor, which obtains a digital representation of the value of the PES from a selected combination of the input digital representations of the analog burst signals. The microprocessor then compares the value of the PES to a desired value indicative of the desired position of the head relative to the selected track and issues a digital correction signal to the power amplifier, which in turn provides an analog current to the actuator coil to adjust the position of the head accordingly.
It follows that an important consideration in digital servo systems is accurately determining the relationship between the value of the PES and the corresponding distance the head is from a known position, for example the center of a track, in order to effect accurate control of the head position. It has been found to be generally desirable to configure the servo system so as to generate a nominally linear PES over the width of a track.
It will be recognized that accurate control of the position of the heads is of paramount importance in the reliable reading and writing of data to the discs. The servo loop generally attempts to maintain the head over the center of the selected track so as to minimize the potential for overwriting data on adjacent tracks or having the magnetization of adjacent tracks interfere with the reading of the data stored on the selected track. Thus, it is common during read and write operations to compare the absolute value of each PES sample to a predetermined safe-threshold value in order to assure the head is correctly positioned relative to the track. Should the value of the PES for a particular sample exceed the threshold, the read or write operation is temporarily suspended until the PES is brought back down to a safe value.
There are generally two reasons why a selected PES sample will have a value which exceeds the safe-threshold value during a read or write operation: either the head is actually positioned at a location sufficiently distant from the center of the track so that the threshold value is exceeded, or a defect exists in the servo information so that the head is correctly positioned, but the reported PES sample is erroneous.
As to the first reason, it will be recognized that mechanical shocks supplied to the disc drive during operation can result in movement of the head away from the center of the selected track (sometimes referred to as an off-track condition). As a result, it is desirable to suspend the read or write operation while the head is in such an off-track condition.
As to the second reason, however, it will be recognized that defects can exist in the servo information written to the discs during manufacture of the disc drive. Such defects in the servo information can occur as a result of a localized anomaly in the media on the surface of a disc, so that the media does not possess the necessary magnetic properties to allow the servo information to be written at this location. Additionally, errors can also occur during the servo track writing process during manufacture of the disc drive, so that incorrect servo information is provided to the disc at a particular location.
Regardless of the source of the defect in the servo information, such a defect is manifested as a one sample error in the PES. The erroneous PES sample does not provide a true indication of head position relative to the center of the selected track, and further, if the erroneous PES sample is interpreted by the servo loop as an impulse function, an unwanted oscillatory response will be induced into the system.
Because of the problems associated with defects in the servo information, it is desirable to provide a servo loop which is capable of determining when an off-track condition is caused by a true positioning problem (and make the necessary position corrections) and when an off-track condition is caused by a servo defect (and ignore the erroneous PES sample). However, prior art servo loops have heretofore been generally unsuccessful in distinguishing excursions in the PES that are caused by servo defects from those caused by external shocks to a drive. This is particularly true in servo loops which rely on the PES value to identify off-track conditions.
Further, a marginal (relatively small) servo defect can cause a PES excursion that does not exceed an off-track threshold, but still lowers the ability of the drive to detect off-track write faults. As a result, a marginal servo defect may not be detected in a non-shock environment or at the beginning of life for the drive, but low levels of vibration or read performance degradation as a result of age or temperature can lead to unexpected, subsequent errors during the life of the drive.
There is a need, therefore, for an improved approach to detecting servo defects in a disc drive, such approach distinguishing erroneous PES samples which do not give true indications of head position from true PES samples which indicate actual step-impulse movements of the head as a result of, for example, mechanical shocks to the drive.